36 SCIENTIFIC REPORTS OF THE AGRICULTURAL RESEARCH 



entirely agree with observed facts. The retention of phos- 

 phoric acid by the soil is not satisfactorily explained on 

 the purely chemical grounds and, in all probability, the 

 physical phenomenon of adsorption is an important 

 factor. Adsorption is an instantaneous process, and con- 

 sequently under conditions which obviate any possibility of 

 the formation of insoluble compounds, may be looked upon 

 as the factor governing the range of distribution of the 

 phosphoric acid through the soil and the availability of the 

 retained phosphate. On the other hand, the introduction 

 of a secondary factor such as the formation of insoluble 

 calcium phosphates must considerably modify the result, but 

 the magnitude of this modification must depend largely 

 upon the rapidity of the reaction concerned. It is con- 

 ceivable that if, for instance, the rate' of reaction between 

 monocalcic phosphate — the chief constituent of super- 

 phosphate — and calcium carbonate is very rapid the range 

 of distribution through a soil will be limited. Conversely, 

 given a very slow rate of reaction the distribution will be 

 wider. Evidence on this point is conflicting and it is im- 

 possible to formulate with any degree of certainty the 

 probable course of events in highly calcareous soils such as 

 are found in Bihar and other portions of the Gangetic 

 alluvium. It therefore appeared desirable to investigate 

 the predominating factors governing the distribution of 

 soluble phosphates through calcareous and non-calcareous 

 soils. 



A study of the reaction between calcium carbonate and 

 monocalcic phosphate showed that this is an extremely 

 rapid process resulting in the formation of the compara- 

 tively insoluble dicalcic phosphate. The latter in turn 

 slowly reacts with further quantities of calcium carbonate 

 forming tricalcic phosphate. During the course of reac- 

 tion carbon dioxide gas is produced which by increasing 

 the number of calcium ions in solution reduces very 

 materially the quantity of phosphoric acid formed in solu- 

 tion. Consequently, with calcareous soils the importance 

 of cultural conditions which will tend to keep the C0 2 con- 



